Assessing redox zones and seawater intrusion in a coastal aquifer in South Korea using hydrogeological, chemical and isotopic approaches

- We examined distinct patterns of redox zoning in coastal shallow groundwater.
- Significant sulfate reduction occurs locally with or without methanogenesis.
- Methanogenesis zone overlies the BSR zone at deep seaward sites.
- Considerable BSR in clay layer resulted in high δ34S values (up to 99.9‰).

A shallow (< 25 m), coastal alluvial groundwater system underneath a paddy field in the Yangyang area of South Korea was investigated to examine the occurrence of redox processes. The aquifer is affected by seawater intrusion, and is characterized by a highly reducing environment facilitated by high organic matter in the sediments. Hydrochemical data with δ34S and δ18O of sulfate were examined for depth-specific groundwater from two multilevel samplers that were installed at seaward (YY2) and landward (YY1) locations. Shallow groundwater showed distinct patterns of redox zoning. Evidence of significant bacterial sulfate reduction (BSR) was observed throughout the nearly entire depths of the two boreholes, while at the depths of active seawater intrusion in YY2, conditions suitable for methanogenesis were never reached. Thus, at YY2 the deep zone of intense BSR was overlain by a zone in which methanogenesis occurred in a low-sulfate environment. In contrast, concurrent BSR and methanogenesis in YY1 occurred at depths with high sedimentary organic matter and low dissolved sulfate due to intensive BSR. Considerable BSR in the groundwater representing trapped seawater in a clay layer had resulted in a very strong increase of δ34Ssulfate up to 99.9‰. The inferred sulfur isotopic enrichment factor (ε) for BSR in the lower part of YY2 was − 12.3‰, while ε at YY1 was much higher (− 45.9‰). In addition, the observed trends of δ18Osulfate at YY1 indicated significant oxygen isotope exchange of sulfate-oxygen with ambient water, likely because of lower cell-specific rates of BSR and higher sulfur isotope fractionation as indicated by the δ34S. In contrast, there was little evidence of oxygen isotope exchange between water and SO42−  at YY2. This study indicates that in coastal aquifers with sulfate-reducing activity, δ34S and δ18O of sulfate can reveal zones of active seawater intrusion and of trapped seawater. This study provides an example of the application of sulfur and oxygen isotope data with hydrochemical and hydrogeologic data to interpret complex redox zonation in an organic-rich coastal environment.